Re-grown n-type gallium nitride (GaN) contact layers are becoming increasingly important, particularly for highly-scaled, high-speed GaN high electron mobility transistor (HEMT) structures. Traditional doping of GaN either with silicon (Si) alone or germanium (Ge) alone results in drawbacks relative to maximum active carrier concentration, atomic solubility, lattice strain and morphology degradation. In particular, achieving a relatively high maximum active carrier concentration along with maintaining a relatively a smooth surface/interface morphology between a traditionally doped layer and other epitaxial layers remains elusive. As such, a need remains for providing a HEMT device having an extended upper limit of n-type active carrier concentration in a layer of the HEMT device while maintaining a smooth surface/interface morphology between the layer and other epitaxial layers.